From Liquipedia Starcraft 2 Wiki
The Assimilator, a Protoss structure that can only be built on top of Vespene Geysers, allows the player to collect Vespene Gas. Probes sent to an Assimilator will return Vespene Gas to the nearest Nexus. The Assimilator is equivalent to the Terran Refinery and the Zerg Extractor, but has significantly higher endurance.
 Competitive Usage
The most efficient way to harvest Vespene from an Assimilator is to assign exactly three Probes to it. Selecting the Assimilator allows the player to see how many Probes are currently assigned.
The timing of when to build Assimilators and transfer Probes from mineral gathering to Vespene is an important strategic consideration. Many Build Orders will specify when to build the first or second Assimilator, and the player must also carefully balance mineral and gas needs during mid-game transitions.
Like its Terran and Zerg counterparts, players will sometimes build Assimilators on the Vespene Geysers in their opponents' base, usually in the early stages of a match. This maneuver, called a "Gas Steal," provides several competitive benefits including a delay in the opponent's ability to gather Vespene. The Probe's ability to begin warping in an Assimilator and then immediately continue scouting makes the gas steal somewhat easier for Protoss players.